The elevated plus maze (EPM) was established to test anxiety in rodents. Although there are various types of the arm structures in EPMs, with or without ledges along the open arms or with transparent or opaque walls along the closed arms, the combined effects of these structures on the anxiety-like behavior in an EPM have not been well studied. We compared anxiety-like behavior in 5 weeks-old Sprague-Dawley rats by measuring exploration onto open arms in EPM tests on 4 types of EPM apparatus: open arms with or without ledges Χ closed arms with opaque or transparent walls. The absence of open arm ledges significantly reduced open arm exploration. Furthermore, the shortest exploration time in the distal segment was in apparatus with no-ledges open arms and opaque walls closed arms (NL/O), whereas the longest time was in the apparatus with ledges open arm and transparent walls closed arms (L/T). These results clearly showed that the absence of ledges along the open arms significantly increases anxiety-like behavior and suggested that the NL/O and L/T combinations induces the highest and lowest emotional reactivity, respectively. Therefore a subsequent experiment was performed using the lowest effective dosage of diazepam treatment in order to compare detection sensitivity between the NL/O and L/T apparatus types. We could observe the effect of 0.5 mg/kg diazepam in the NL/O apparatus, but not in the L/T apparatus at the same dosage. In conclusion, these results indicated that the arm structure of the apparatus significantly influenced rat EPM behavior, and that the NL/O type of EPM apparatus possessed a higher detection sensitivity than the L/T type.